Thomlinson said Microsoft was being targeted by "determined and persistent adversaries."

He didn't specify who those were, but many have linked the recent attacks on Internet companies with intrustions by Chinese hackers into the systems of publishers like the New York Times and Bloomberg.

It's not clear, however, that the recent attacks originated from China.

iPhoneDevSDK recently told users that it discovered that an administrative account on its site had been hacked, which allowed hackers to inject infectious code into its Web pages. The site says it believes its software is otherwise secure.